Of course observationally it is the images that are detected and not the source itself. Although finding the distance \(D_{ds}\)  to the source from its redshift
is possible under certain circumstances [1]}, the angular position \(\beta \)  is not directly observable. We can, however, adapt a scheme developed for
ECG to find \(\beta \)  from primary and secondary image positions, time delays and their differential time delays to EQG.
In Fig. REF a we plot \(\theta _{p}\)  and \(|\theta _{s}|\) , the respective primary and secondary angular image positions
in GR and EQG for \(\mathcal {D}=0.5\) . Each of these lines crosses both the plot of GR and EQG. We do not know if the theory governing the strong gravitational field is GR or EQG (assuming that one or the other is the empirically correct theory). However the correct theory must (for a given set of parameters) have the same value of \(\beta \)  at both intersection points, allowing for its determination.
